1. Field of the Invention
This invention relates generally to a method and device to deliver successive pieces of source material to a material heating element in a vacuum deposition apparatus, and particularly to a method and device for replenishing the filament of a vacuum deposition apparatus with source material without opening the apparatus to the atmosphere.
2. History of the Prior Art
The deposition of selected pure metals or other materials possessing particularly desirable properties, generally in the form of very thin films, is a common but very important step in the production of miniaturized electronic components. In fact, it is not uncommon to have the deposition of mor than one material in successive thin film layers on the same substrate, with or without the deliberate but partial elimination of a preceding film before a successive film is deposited thereon.
In the art of vacuum deposition of such source materials on selected substrates, there are several methods of obtaining a flux of vaporized source material atoms. These methods include a heating element heated by its own electrical resistance, heated crucibles or boats that are either self-heated or heated by external means, electron beam vaporization sources, sputtering sources, RF-heated sources, inductively-heated sources, and the like. Each of these techniques has its own advantages and disadvantages, depending primarily upon the geometry and orientation of the substrate, the type of substrate, the source material itself, the coating thickness, the duration for which the coating has to be applied, the purity of the coating material, and the like.
In the field of vapor deposition of thin films using heated filament sources, different geometries are found to be useful in dealing with different types of source material. One solution, particularly suited for the vaporization of aluminum from a tungsten filament, utilizes a multistrand, twined, helical filament for heating the source material. On the other hand, gold can be easily evaporated from a tungsten filament constructed from several turns of solid wire. Where clean vapor deposition of very thin films is desired, e.g., in the tuning of quartz crystal oscillators, it is important that only vapor and no solid or liquid particles should reach the substrate from the source. Such particles may arise, for instance, when gas bubbles escape from the filament or from the source material being vaporized. Precleaning of the heating element, preferably by heating it in a high vacuum, is recommended. Thus, minimizing the exposure of the deposition apparatus and the heating element to air during replenishment of the source material is paramount.
Typically, when the source material is exhausted, vacuum is broken, and another piece of source material is positioned by the operator onto the heating element. The entire system then has to be evacuated and degassed, brought up to working conditions, and the process repeated each time the source material is exhausted. Numerous alternatives, all aimed at providing a more or less continuous supply of source material for vaporization, have been proposed. Each, however, has certain limitations and problems, and it is highly desirable to be able to utilize a simple helical coil heating source to which source material is periodically added without totally disrupting the process. Since substrates are often coated on both sides it is also highly desirable that the replenishment device be small, easy to operate, and one that does not require the location or addition of vacuum ports at awkward positions in the overall apparatus.
Some vacuum deposition apparatus provide for a source material in the form of a wire which is continually fed onto a heated filament. In such an arrangement, the entire feed mechanism must be electrically insulated from the deposition chamber, so as not to "short out" the heating filament. In operation, however, the heating filament is generally nearly white hot and somewhat fragile, so that continuous feeding could result in destruction of the filament. Furthermore, a continuously fed wire would tend to act as a heat sink, so that it could be difficult to continually supply sufficient energy to keep the heating element hot enough to perform its function adequately.